1. Field of the Invention
The present invention relates to a document reader, or particularly to a document reader so as to read a document in an image forming apparatus such as a copying machine.
2. Description of the Prior Art
The principal part of a copying machine with a document reader is shown in FIG. 1 and FIG. 2.
One of the documents set on the document tray 12 is uptaken in the body of the copying machine 1 by a feed roller 13, and is carried to the fixed reading position on the document glass plate 5 by a feed in/out roller 14. Such feed roller 13 and feed in/out roller 14 are actuated by ADF motor 17.
The document is read by one-way movement of a reading unit 4 along the underside of the document glass plate 5 so as to scan the overall document. The reading unit 4 is moved by means of a scan motor 23, e.g. servo-motor or stepping motor. After reading, the document is fed out from the document glass plate 5 into a document assist tray 16 by the reversing actuation of the feed in/out roller 14. Afterwards the reading unit 4 returns to its home position.
A main control circuit 2 of the copying machine 1 detects the switch-status of a paper-feed resist switch 3, TIM switch 25, and HP switch 26. In accordance with the results of each switch-status, the main control circuit 2 controls an ADF motor 17 by means of a ADF motor control circuit 18 provided between them, and controls a scan motor 23 by interstice of a scan motor control circuit 24 between them.
The power, making the ADF motor 17 and the scan motor 23 actuate, is supplied to them from +24 V power source 8 in a power circuit 7.
Means for feeding in/out a document (as described ADF hereinafter) is generally composed of the document tray 12--the ADF motor control circuit 18 and common components, e.g. the main control circuit 2 and the power circuit 7, and means for scanning/returning is generally composed of the scan motor 23--HP switch 26 and said common components.
Now the scan motor 23 is controlled in accordance with the procedure of a flowchart shown in FIG. 5 when a reading unit 4 of a conventional document reader is returning to its home position. In the first place, it is judged whether the ADF is driven not to feed in and out a document (L1). The scan motor 23 is driven in the reversal rotation at a relative high speed non-actuation of the ADF (L3). Conversely, during actuation of the ADF, the actuation order signal is sent to it (L2).
After that, the scan motor 23 is driven in the reversal rotation at said speed (L3). At this time, the scanning speed V.sub.1 of the reading unit 4, for instance about 300 mm/sec, is approximately equal to three times the speed in scanning a document at an equi-magnification copying.
After the reading unit 4 arrives at the TIM switch 25 (L4), the scan motor 23 is driven for normal rotation in a short time to brake the reading unit 4 (L5). Next the scan motor 23 is driven in the reversal rotation at a lower speed than that hitherto (L6). The scanning speed V.sub.2, for instance about 100 mm/sec, is nearly equal to the speed in scanning a document at an equi-magnification copying at this time.
The scan motor 23 is driven again in the normal rotation for a short time to brake the reading unit 4 (L8) after the HP switch 26 detects a return of the reading unit 4 to its home position (L7). Afterwards the scan motor 23 should stop (L9).
As previously noted on the conventional document reader, the return speed of the reading unit 4 is independent of the actuation of the ADF.
In regard to the copying machine 1 mentioned above, the power is supplied to the ADF motor 17 and the scan motor 23 from +24 V power source 8.
The current for the return period t.sub.2 -t.sub.4 increases as shown in FIG. 6.
Namely the current increases slightly as a broken line shown in FIG. 6 during high speed driving of only the scan motor 23 without a driving of the ADF motor 17, or increases markedly as a solid line in FIG. 6 during a high speed driving of the scan motor 23 with a driving of the ADF motor 17.
Therefore it causes a problem that +24 V power source 8 of large capacity is required for the case when high speed driving of the scan motor 23 with a driving of the ADF motor 17 occurs. For this reason, it also causes the problem that a loss of the power consumption becomes large.